Thus far, a large number of proposals have been made in relation to technology for capturing diagnostic images using light, and one of these proposals is photoacoustic tomography (called PAT below). An apparatus using PAT is particularly useful in diagnosis of skin cancer or breast cancer, and there are high expectations for the use of this kind of apparatus as a medical device in place of an ultrasound diagnostic apparatus, an X-ray apparatus, or an MRI apparatus, which have been used conventionally for such diagnosis.
The basic measurement principle of PAT is based on that of photoacoustic analysis technology, such as that disclosed in Patent Literature 1 (see below). According to the technology disclosed in Patent Literature 1, measurement light produced by a light source is directed onto a specimen via an irradiation optical system, and a photoacoustic wave from the specimen is received by one ultrasound probe, whereby it is possible to measure photoacoustic characteristics of the specimen.
If measurement light, such as visible light or near-infrared light, or the like, is irradiated onto living tissue, a light absorbing material inside the living organism, and in particular, material such as hemoglobin in the blood, and the like, absorbs energy from the measurement light and generates a photoacoustic wave as a result of this. In PAT, information about the living tissue is visualized by measuring this photoacoustic wave. With PAT technology, it is possible to achieve quantitative measurement, and also three-dimensional measurement, of the density distribution of light energy absorption, in other words, the density distribution of light absorbing material, in the living organism.
Moreover, by combination with ultrasound diagnostic technology using ultrasound echo, it is possible simultaneously to measure distribution information about internal tissue of a living organism based on a photoacoustic analysis method, and information about the morphological characteristics of the internal tissue of the living organism by ultrasound diagnostic technology, which is suitable for accurate diagnosis through more precise characterization of diagnostic results.
In general, in the diagnosis of breast cancer, an overall pass/fail diagnosis is made on the basis of the results of palpation or the plurality of modalities described above. The key bases for this diagnosis are image diagnostic results which indicate the presence or absence of new blood vessels, which are produced by cancer. A photoacoustic image obtained in relation to breast cancer which has an increased blood flow compared to normal tissue, due to the new blood vessels, has the potential for providing superior detection capability compared to measurement by a conventional ultrasound diagnostic apparatus or an X-ray apparatus or MRI apparatus, or the like.
Moreover, PAT has a significant benefit in terms of the burden on the patient, since light is used for capturing a diagnostic image and therefore image diagnosis by radiation-free, non-invasive means is possible, and so may offer advances in screening and early diagnosis of breast cancer, compared to an X-ray apparatus, which cannot readily be used for repeated diagnosis, due to problems of radiation exposure.
With PAT which measures a photoacoustic wave produced as a result of a light absorbing material absorbing energy from measurement light, it is necessary to control the recording of the photoacoustic signal in synchronism with the irradiation of the tissue with the measurement light. In a solid-state laser, which is generally used as a light source, time lag (no more than 1 microsecond) and fluctuation (several tens of nsec approximately) occur from the input of a laser emission control signal until the actual emission of laser light, and therefore a method for achieving synchronism by detecting the measurement light is generally employed. To detect the measurement light, an optical composition is required which divides off a portion of the measurement light and guides the divided light portion to an optical sensor. Furthermore, the optical sensor used is required to have high-speed response which enables the measurement light to be measured as a pulse.